Front surface fans: blow upwards or downwards?

[Roel]

For cooling the primary I'll install 3 fans: 1 on the backside of the mirror, 2 that will blow cool air over the front surface.

What would be the best direction for the airflow. Upwards or downwards? (Dob seen in horizontal position)

I can imagine upwards is better, because warm air allready tends to rise upwards and the air is coolest near the ground. I don't want that downward blown air to sneak up in the OTA for the second time. But what about dew? Will the air near the ground be more damp so the mirror will get dew on it faster?

(Sideways is not an option, the altitude bearings will obstruct the fan's holes.)

[brantuk]

Upwards is best - if the question relates to your dob then a shroud will also help the dew problem (helps block stray sideways light out too). If it's particularly dewey where you view from then a dew control system may need installing.

[dweller25]

I have the fan placed behind the primary with a baffle to ensure all the fresh cool air goes up the tube. This does three things simultaneously :-

1. Cools the primary mirror.

2. Breaks down the boundary layer on the surface of the primary.

3. Mixes the cooler air at the side of the tube with the warmer air in the middle of the tube and pushes it out of the front of the scope.

The one fan with rear baffle is very effective and I use it during viewing too.

HTH

[Roel]

A shroud and and a secondary heater will indeed be installed.

With previous Dobs I never had a dewed primary, but I have no idea if forced front cooling will make the mirror dew quicker when it's been cooled to below ambient temperature. Probably not, because the moving air with prevent the dew to form in the first place, but I'm not an expert on this matter...

[yesyes]

This is something I've been wondering about. I read about tube currents that distort/blur the viewing through the scope. Does installing a fan and the resulting air flow not have the same (negative) effect? Or would you switch off the fan once the mirror has cooled down?

[dweller25]

There is no doubt in my mind that using a fan even during observing has a very positive effect. I always thought the varying image quality was poor seeing but once I fitted the fan the "seeing" problems went away. The fan on my newt is the best £5 I ever spent.